If Deep Learning is non convex, then why use convex loss?
Well, you are definitely mixing two different things. Here are those bits:


The function that deep learning approximates is basically a function that best fits the INPUT DATA points. You should not think about its differentiability or optimization aspects. We don't care what type of function it is; we just want the best fit of input data (ofcourse overfitting is not desired though). So, the space in which this fucntion lies has dimensions equal to the dimensions of input data (or number of input features). For every function fit we get some loss which basically is the distance from actual data point from the one predicted by the function fitted.
Loss function is the one which is defined in terms weights and biases. You can think the space in which loss function lies has dimensions equal to the number of PARAMETERS to tune (weights and biases). So, this space is very different from the one described in the first point. Now, here we do care about the differentiability of the function because we want a point where the function outputs least value (minimize the loss) given some particular values of inputs (weights and biases in this case) and by differentiating we can traverse to that "optimal" point in the function.


Hope this clears your doubt.